Method for operating an internal combustion engine of a vehicle, in particular a motor vehicle

ABSTRACT

The invention relates to a method for operating an internal combustion engine of a vehicle, in particular a motor vehicle, with a first operating range as the lean operating range, to discharge the nitrogen oxide storage catalyst by means of an engine control device switching from the lean operating range to the rich operating range taking place. As claimed in the invention, the engine control device blocks switching into the lean operating range if the additional amount of fuel consumption for discharges in a certain, definable evaluation interval which extends over several lean operating phases is greater than or equal to the reduced amount of fuel consumption by lean operation in this monitoring interval. Furthermore, the engine control device enables lean operation and thus switching between the lean operating range and the homogeneous operating range if the additional amount of fuel consumption for discharges in the evaluation interval is smaller than the reduced amount of fuel consumption by lean operation in this evaluation interval. The reduced amount of fuel consumption is determined as a function of the raw mass flow value of the nitrogen oxide averaged over the evaluation interval, as a function of the amount of fuel saved which has been averaged over the evaluation time interval in the lean operating phases which occur in the evaluation interval compared to the homogeneous operating range phases in this evaluation interval, and as a function of the time between two torque requirements which exceed a definable load boundary value and/or rpm boundary value and which cause departure from the lean operating range, which time has been averaged over the evaluation interval, while the additional amount of fuel consumption is determined as a function of a storage catalyst charging state averaged over the evaluation interval.

The invention relates to a method for operating an internal combustionengine of a vehicle, in particular a motor vehicle as claimed in thepreamble of claim 1.

In current automotive engineering, spark ignition engines as internalcombustion engines with direct gasoline injection are preferred overconventional manifold injection, since these internal combustion enginescompared to conventional spark ignition engines have distinctly moredynamics, are superior with respect to torque and output, and at thesame time enable a reduction in fuel consumption by up to 15%. This ismade possible by so-called stratification in the partial load range, inwhich an ignitable mixture is required only in the area of the sparkplug, while the remaining combustion chamber is filled with air. Sinceconventional internal combustion engines which operate according to themanifold principle can no longer be ignited at such a high air excess asis present in direct gasoline injection, in this stratification mode thefuel mixture is concentrated around the spark plug which is positionedcentrally in the combustion chamber, while pure air is present in theedge areas of the combustion chamber. In order to be able to center thefuel mixture around the central spark plug positioned in the combustionchamber, a dedicated air flow in the combustion chamber is necessary, aso-called tumble flow. An intensive, roller-shaped flow is formed forthat purpose and fuel is injected only in the last third of the upwardmotion of the piston. The combination of the special air flow and thededicated geometry of the piston which has for example a pronounced fuelflow depression, concentrates the especially finely atomized fuel in aso-called “mixture ball” optimally around the spark plug and reliablyignites it. The engine control or engine control device provides for therespectively optimum adjustment of injection parameters (injectioninstant, fuel pressure).

These internal combustion engines can therefore be operated for acorrespondingly long time in lean operation; this overall has abeneficial effect on fuel consumption, as has already been described inthe foregoing. This lean operation however entails the disadvantage of amuch greater amount of nitrogen oxide in the exhaust gas so that thenitrogen oxides (NOx) in the lean exhaust gas can no longer becompletely reduced with a three-way catalyst. In order to keep nitrogenoxide emissions within prescribed limits, for example the Euro-IVboundary value, nitrogen oxide storage catalysts are used additionallyin conjunction with such internal combustion engines. These nitrogenoxide storage catalysts are operated such that in them the large amountsof nitrogen oxides which are produced by the internal combustion engineare stored. As the amount of stored nitrogen oxides increases, asaturation state in the nitrogen oxide storage catalyst is reached sothat the nitrogen oxide storage catalyst must be discharged. To thisend, for a so-called discharge phase, switching takes place briefly tosubstoichiometric, rich engine operation by means of the engine controlor engine control device, in which the internal combustion engine isoperated with a rich mixture which has a shortage of air. At the startof this discharge phase, the oxygen reservoir of the nitrogen oxidestorage catalyst is generally emptied, by which the oxygen which isnecessary for the discharge process is made available. In this dischargeprocess the stored nitrogen oxide is reduced to nitrogen (N₂) especiallyby the hydrocarbons (HC) and carbon monoxide (CO) which are present in alarge amounts under these rich operating conditions; this nitrogen canthen be released into the environment. Operating the internal combustionengine of a motor vehicle in a first operating range as the leanoperating range is already known in general; in this first operatingrange the internal combustion engine is operated with a lean mixturewhich has an air excess and thus an oxygen excess, and the nitrogenoxides produced by the internal combustion engine are stored in anitrogen oxide storage catalyst, to discharge the nitrogen oxide storagecatalyst by means of an engine control device switching from the leanoperating range to the rich operating range taking place, in which theinternal combustion engine is operated with a rich mixture which has ashortage of air and in which the nitrogen oxides stored in the nitrogenoxide storage catalyst during the lean operating range are dischargedfrom the nitrogen oxide storage catalyst. Furthermore there is a secondoperating range as a homogeneous operating range in which the internalcombustion engine is operated with an essentially stoichiometrichomogeneous mixture (lambda=1), switching between the lean operatingrange and the homogeneous operating range being undertaken by the enginecontrol device depending on the operation-dictated load requirementand/or rpm requirement when a definable switching condition is reached,and switching taking place by the engine control device into the richoperating range first for discharge of the nitrogen oxide storagecatalyst before switching from the lean operating range to thehomogeneous operating range. Specifically, the lean operating range inthis instance is for example a stratified one in conjunction with adynamic driving style, as is the case for example in city driving,switching generally takes place by the engine control device based onthe lean operating range in which the lambda value is approximately 1.4,in particular based on the operation-dictated increased load requirementand/or rpm requirement, into the homogenous operating range, in whichthe internal combustion engine is operated essentially with astoichiometric homogeneous mixture of lambda=1. Before switching intothe homogeneous operating range, the engine control device switchesfirst into the rich operating range in order to discharge the nitrogenoxide storage catalyst. Research has shown that in this operating mode,in spite of temporary lean operation, the theoretical lean operationfuel savings potential which is actually present is not fully exhausted.Another problem here is that in a very dynamic driving style it isnecessary to depart from the lean operating range due to an increaseddemand for torque more often under certain circumstances, by which theneach time there is a need for nitrogen oxide storage catalyst discharge,i.e., a rich operating phase. This also leads to increased fuelconsumption.

Similar process guidance is known from the generic DE 100 64 279 A1, inwhich, depending on the deterioration of the exhaust gas composition,switching takes place between lean, rich and homogeneous operation. Theswitching decision is made depending on the deterioration of the storagecapacity of the nitrogen oxide storage catalyst which is designated asthe NO_(x) absorption means. In particular, when a deterioration of theefficiency of the nitrogen oxide storage catalyst is ascertained, leanoperation which is designated as an oxygen excess-air-fuel ratiooperation is to be blocked.

DE 197 53 718 C1 discloses a process for operating a diesel engine whichcomprises an engine Control which controls operation of the dieselengine depending on the engine characteristics and which enablesrich/lean control of the diesel engine. The engine control comprises acomputer which effects switching to rich or lean operation of the dieselengine depending on predetermined switching criteria. Furthermore, thereare sensors which communicate with the computer and which monitor theparameters are necessary for the switching criteria, and a memory whichcommunicates with the computer, in which the engine characteristics arestored for operation of the diesel engine. The computer effectsswitching from lean to rich operation when the maintenance of aregeneration temperature of the storage catalyst element through whichthe exhaust gases of the diesel engine have flowed and the presence of apredetermined charging state of the storage catalyst element throughwhich the exhaust gases of the diesel engine have flowed are satisfiedas the switching criteria. Furthermore, the computer effects switchingback from rich to lean operation when one of the switching criteria forswitching from lean to rich operation is not present or a regenerationtime has elapsed which depends on the respective charging state of thestorage catalyst element through which the exhaust gases of the dieselengine have flowed at the start of the rich operating phase, or there isa predetermined content of the reducing agent in the exhaust gasesdownstream from the storage catalyst element or the exhaust gastemperature is below a predetermined threshold value.

Furthermore, in the dissertation of Andreas Hertzberg (Stuttgart 2001)entitled “Operating strategies for a spark ignition engine with directinjection and a NO_(x) storage catalyst” in Chapter 6, especially underitem 6.4.2, tests on operation of an internal combustion engine in leanoperation are described and evaluated. Here the focus was especially onthe consumption difference of various test driving cycles depending ontorque threshold values.

The object of the invention is therefore to make available analternative process for operating the internal combustion engine of avehicle, in particular a motor vehicle, with which an operating mode ofthe internal combustion engine which has been optimized with respect tofuel consumption, especially by optimized lean operation, becomes easilypossible.

This object is achieved with the features specified in claim 1.

As claimed in claim 1, the engine control device blocks switching intothe lean operating range if the additional amount of fuel consumptionfor discharges in a certain, definable evaluation interval which extendsover several lean operating phases is greater than or equal to thereduced amount of fuel consumption by lean operation in this evaluationinterval. Furthermore the engine control device enables lean operationand thus switching between the lean operating range and the homogeneousoperating range if the additional amount of fuel consumption fordischarges in the evaluation interval is smaller than the reduced amountof fuel consumption by lean operation in this evaluation interval. Thereduced amount of fuel consumption is determined as a function of theraw mass flow value of the nitrogen oxide averaged over the evaluationinterval, as a function of the amount of fuel saved which has beenaveraged over the evaluation time interval in the lean operating phaseswhich occur in the evaluation interval compared to the homogeneousoperating range phases, and as a function of the time between two torquerequirements which exceed a definable load boundary value and/or rpmboundary value and which cause departure from the lean operating range,which time has been averaged over the evaluation interval. Furthermore,the additional amount of fuel consumption is determined as a function ofa storage catalyst charging state averaged over the evaluation interval.

Advantageously, in this operation of an internal combustion engine thedriving behavior of the driver can be “learned” and thus a predictioncan be made with respect to probable future driving behavior. That is,in this operating mode the driving behavior in the past is evaluatedover a reasonable evaluation interval and based on this evaluation aprediction for the future, i.e., for the presumed lean operating time,can be computed. In contrast to a purely steady-state approach, in thisapproach which is referenced to the evaluation interval on average, hereif necessary the lean operating range is thus not enabled even if thiswould occur according to a purely steady-state standpoint at a certaintime, since the driving behavior and not the current steady-stateoperating point is now taken into account overall as claimed in theinvention over the averaged values by the approach and the focus on areasonable time window.

Consequently, an especially optimized operating mode, especially withrespect to fuel savings by lean operation, is possible overall.

As a result, the lean operation fuel savings potential is fullyexhausted since switching into the lean operating range is carried outonly when this is reasonable based on the driving behavior of thedriver, i.e., it may entail fuel savings. As soon as the engine controldevice recognizes that this is not the case, the homogeneous operatingrange is chosen. The evaluation interval is especially advantageously atleast approximately 100 seconds.

According to the especially preferred process guidance as claimed inclaim 2, provision is made such that the additional amount of fuelconsumption which is caused by the rich operating phases in theevaluation interval is computed as the sum of a first amount of fuelwhich is required for discharge of the oxygen reservoir and a secondamount of fuel which is required for discharge of the nitrogen oxidereservoir. The first amount of fuel, i.e., the amount of fuel fordischarging the oxygen reservoir, is thus more or less constant per leanoperating phase, while the second amount of fuel is mainly a function ofthe raw nitrogen oxide emissions during the lean time, so that thesecond amount of fuel is averaged over the evaluation interval, by whichthe additional amount of fuel consumption can be easily determined as afunction of the storage catalyst charging state averaged over theevaluation interval. Since lean operation is run with an excess ofoxygen, the oxygen reservoir of the nitrogen oxide storage catalyst isvery quickly completely charged so that the oxygen charging of thenitrogen oxide storage catalysts over the lean phase can always beregarded as more or less constant. The nitrogen oxide charging of thenitrogen oxide storage catalyst is conversely mainly a function of thelean time and optionally also of the raw nitrogen oxide mass flow. Forexample, for regeneration of 1 g of oxygen an amount of fuel ofapproximately 0.23 g is necessary, while for regeneration of 1 g ofnitrogen dioxide approximately 0.15 g of fuel are necessary.

As claimed in claim 3, provision is made such that the first lean timeis computed from the quotient of the current nitrogen oxide storagecapacity of the nitrogen oxide storage catalyst and the averaged rawnitrogen oxide mass flow value. The averaged time between two torquerequirements which exceed a definable load boundary value and/or rpmboundary value and which cause departure from the lean operating rangeas the second lean time is compared to the first lean time, the minimumor the shorter of the two lean times then being multiplied by theaveraged amount of fuel saved in the evaluation interval. In this way,the reduced amount of fuel consumption in the evaluation interval can bedetermined especially easily. With this process guidance an especiallysimple and reliable prediction of the driving dynamics and thus also aconclusion about future driving behavior are possible, so that optimizedoperation of the internal combustion engine, especially optimization ofthe lean operating phases, becomes possible.

By special preference, as claimed in claim 4, the current nitrogen oxidestorage capacity amount of the nitrogen oxide storage catalyst can bedetermined as a function of the temperature and/or the ageing stateand/or sulfurization.

Specifically, as claimed in claim 5, provision is made such that thenitrogen oxide mass flow upstream from the nitrogen oxide storagecatalyst and/or the nitrogen oxide mass flow downstream from thenitrogen oxide storage catalyst are each integrated over the same timeinterval, the switching operating point being determined as a functionof the instantaneous operating temperature at the instant of switchingto establish the switching instant from the storage phase to thedischarge phase and thus from the lean operating range to the richoperating range at least from the integral value of the nitrogen oxidemass flow upstream and/or downstream from the storage catalyst and/orthe switching instant when a definable discharge switching condition issatisfied in the first stage for determination of the degree of ageingof the storage catalyst. Then the respective switching operating pointin a second stage for determining the degree of ageing of the storagecatalyst is compared to the definable storage catalyst capacity fieldwhich runs over a temperature window, which is optimized especially withrespect to fuel consumption, and which is formed by a plurality ofindividual operating points for a new and an aged storage catalyst. Inthe process a switching operating point which lies within the storagecatalyst capacity field does not constitute a failure to reach theminimum nitrogen oxide storage capacity, but the change relative to theprevious operating point as a measure of the ageing of the storagecatalyst. A switching operating point which departs from the storagecatalyst capacity field conversely constitutes a failure to reach theminimum nitrogen oxide storage capacity. With this procedure currentdetection of the value of the nitrogen oxide storage capacity of thenitrogen oxide storage catalyst can thus be determined especially easilydepending on the operating point with consideration of the degree ofageing and/or sulfurization of the nitrogen oxide storage catalyst.

As claimed in claim 6, provision is made especially preferably here thatto establish the switching instant from the storage phase to thedischarge phase, the relative nitrogen oxide slip as the differencebetween the nitrogen oxide mass flow which has flowed into the nitrogenoxide storage catalyst and the nitrogen oxide mass flow which has flowedout of the nitrogen oxide storage catalyst is determined relative to thestorage time, such that the quotient of the integral values of thenitrogen oxide mass flow upstream and downstream from the nitrogen oxidestorage catalyst is moreover brought into a relative relationship to thedefinable degree of conversion of the nitrogen oxide which is derivedfrom the exhaust gas boundary value, so that when this definableswitching condition is present switching from the storage phase to thedischarge phase is carried out at the switching instant which has beenoptimized with respect to fuel consumption and storage potential.

As claimed in claim 7, provision furthermore is made such that thestorage catalyst capacity field is limited relative to the temperaturewindow on the one hand by the boundary line for a new storage catalystand on the other hand by the boundary line for an aged storage catalystwhich represents the boundary ageing state. In this instance thetemperature window comprises preferably temperature values betweenapproximately 200° C. and approximately 450° C.

The invention will be described in greater detail with the aid of thedrawings.

FIG. 1 shows a schematic diagram of the amount of fuel savings in leanoperation over time and

FIG. 2 shows a schematic diagram of the dictated relationships of theadditional amount of fuel consumption over time.

FIG. 1 shows the amount of fuel savings in the lean operating range overtime, curve 1 showing the time characteristic of fuel savings during thelean time which can be implemented at maximum. Curve 2 plots theintegral of the amount of fuel savings during this lean time which canbe implemented at maximum. Curve 3 conversely plots the averaged amountof fuel savings which is referenced to time during this lean time whichcan be implemented at maximum.

To determine the reduced amount of fuel consumption, this mean amount offuel savings according to curve 3 must be multiplied by the lean timewhich can be implemented at maximum. To determine the lean time whichcan be implemented at maximum, first the averaged time between twotorque requirements which exceed a definable load boundary value and/orrpm boundary value and which cause departure from the lean operatingrange can be determined. This averaged time is referenced to theevaluation interval, i.e., different exceeding torque requirements arecompared in terms of their time interval, and thus the averaged timevalue is made available. This averaged time between two torquerequirements which exceed a definable load boundary value and/or rpmboundary value and which cause departure from the lean operating rangerepresents a so-called second lean time. The quotient of the currentnitrogen oxide storage capacity of the nitrogen oxide storage catalystand the averages raw mass flow value of nitrogen oxide is determined asthe first lean time. The current nitrogen oxide storage capacity of thenitrogen oxide storage catalyst is thus determined as a function of thetemperature and/or degree of ageing and/or sulfurization. The averagedraw mass flow value of the nitrogen oxide is determined here for theevaluation interval likewise by the engine control device. Then thisfirst lean time is compared to the second lean time, the smaller of thetwo lean times, i.e., the minimum of these two mean times, is used inorder to be multiplied by the averaged amount of fuel savings in theevaluation interval.

To determine the additional amount of fuel consumption the sum for therich phases of the first amount of fuel which is required for dischargeof the oxygen reservoir of the nitrogen oxide storage catalyst, whichrich phases follow the lean phase, and a second amount of fuel which isrequired for discharge of the nitrogen oxide reservoir of the nitrogenoxide storage catalyst is found. This relationship is shown in FIG. 2.FIG. 2 shows that the amount of fuel for discharge of the oxygenreservoir is more or less constant (curve 5), while the second amount offuel for discharge of the nitrogen oxide reservoir (curve 4) is afunction of the lean time, since the oxygen reservoir is more or lesscompletely charged immediately after the start of the lean operatingphase, while the nitrogen oxides are more inert and therefore require alonger time for attachment. This means that depending on the respectivelean operating phase time, more or fewer nitrogen oxides can be storedin the nitrogen oxide reservoir during this lean phase. Curve 6 is thesum of the amounts of fuel of curves 4 and 5. If averaging is also doneagain over time here, i.e., over the evaluation interval,time-referenced nitrogen oxide storage catalyst charging with nitrogenoxides results, so that with simultaneous consideration of the lean timethe additional amount of fuel consumption can be computed using thefollowing formula:Additional amount of fuel consumption (g)=amount of oxygen stored(g)×first percentage amount of fuel+time-referenced, averaged NO_(x)storage amount (g/s)×lean time (s)×second percentage amount of fuel.

The lean time provided here results from the sum of the individual leanoperating times in the evaluation interval.

A comparison of the reduced amount of fuel consumption to the additionalamount of fuel consumption, referenced to the evaluation interval, i.e.,a comparison of curve 2 in FIG. 1 and curve 6 in FIG. 2, thus enables anoperating mode such that the engine control device blocks switching intothe lean operating range if the additional amount of fuel consumptionfor discharges in the evaluation interval under consideration which ispreferably approximately 100 seconds is the same or greater than thereduced amount of fuel consumption by lean operation in this evaluationinterval. If conversely the additional amount of fuel consumption fordischarges is smaller than the reduced amount of fuel consumption bylean operation in this evaluation time interval, the engine controldevice enables lean operation and thus switching between the leanoperating range and the homogeneous operating range.

1. Method for operating an internal combustion engine of a vehiclecomprising the steps of: running said engine in a first operating rangeas the lean operating range, said running comprising operating with alean mixture which has an air excess and thus an oxygen excess, duringwhich nitrogen oxides produced by the internal combustion engine arestored in a nitrogen oxide storage catalyst, to discharge the nitrogenoxide storage catalyst by means of an engine control device switchingfrom the lean operating range to the rich operating range taking place,in which the internal combustion engine is operated with a rich mixturewhich has a shortage of air and in which the nitrogen oxides stored inthe nitrogen oxide storage catalyst during the lean operating range aredischarged from the nitrogen oxide storage catalyst, and with a secondoperating range as a homogenous operating range in which the internalcombustion engine is operated with an essentially stoichiometrichomogenous mixture (lambda=1), switching between the lean operatingrange and the homogeneous operating range being undertaken by the enginecontrol device depending on the operation-dictated load requirementand/or rpm requirement when a definable switching condition is reached,and switching taking place by the engine control device into the richoperating range first for discharge of the nitrogen oxide storagecatalyst before switching from the lean operating range to thehomogeneous operating range, and the engine control device blockingswitching into the lean operating range depending on a definableblocking criterion, blocking switching with the engine control deviceinto the lean operating range if the additional amount of fuelconsumption for discharges in a certain, definable evaluation intervalwhich extends over several lean operating phases is greater than orequal to the reduced amount of fuel consumption by lean operation inthis evaluation interval, enabling lean operation by the engine controldevice and thus switching between the lean operating range and thehomogeneous operating range, if the additional amount of fuelconsumption for discharges in the evaluation interval is smaller thanthe reduced amount of fuel consumption by lean operation in thisevaluation interval, determining the reduced amount of fuel consumptionas a function of the raw mass flow value of the nitrogen oxide averagedover the evaluation interval, as a function of the amount of fuel savedwhich has been averaged over the evaluation time interval in the leanoperating phases which occur in the evaluation interval compared to thehomogeneous operating range phases in this evaluation interval, and as afunction of the time between two torque requirements which exceed adefinable load boundary value and/or rpm boundary value and which causedeparture from the lean operating range, which time has been averagedover the evaluation interval, and determining the additional amount offuel consumption as a function of a storage catalyst charging stateaveraged over the evaluation interval.
 2. The process as claimed inclaim 1, further comprising: computing the additional amount of fuelconsumption which is caused by the rich operating phases in theevaluation interval as the sum of a first amount of fuel which isrequired for discharge of the oxygen reservoir and a second amount offuel which is required for discharge of the nitrogen oxide reservoir,wherein the first amount of fuel is more or less constant per leanoperating phase, and wherein the second amount of fuel is at least afunction of the raw nitrogen oxide emission during the lean time suchthat the second amount of fuel is averaged over the evaluation interval.3. The process as claimed in claim 1, further comprising computing thefirst lean time from the quotient of the current nitrogen oxide storagecapacity amount of the nitrogen oxide storage catalyst and the averagednitrogen oxide raw mass flow value, wherein the averaged time betweentwo torque requirements which exceed a definable load boundary valueand/or rpm boundary value and which cause departure from the leanoperating range as the second lean time is compared to the first leantime such that the shorter of the two lean times is then multiplied bythe amount of fuel saved which has been averaged over the evaluationinterval for determining the reduced amount of fuel consumption in theevaluation interval.
 4. The process as claimed in claim 3, wherein thecurrent nitrogen oxide storage capacity amount of the nitrogen oxidestorage catalyst is determined as a function of the temperature and/orthe ageing state and/or sulfurization.
 5. The process as claimed inclaim 3, wherein the currently detected value of the nitrogen oxidestorage capacity of the nitrogen oxide storage catalyst is determineddepending on the operating point with consideration of the degree ofageing and/or sulfurization of the nitrogen oxide storage catalyst suchthat the nitrogen oxide mass flow upstream from the nitrogen oxidestorage catalyst and/or the nitrogen oxide mass flow downstream from thenitrogen oxide storage catalyst are each integrated over the same timeinterval, wherein to establish the switching instant from the storagephase to the discharge phase and thus from the lean operating range tothe rich operating range at least from the integral value of thenitrogen oxide mass flow upstream and/or downstream from the storagecatalyst and/or the switching instant when a definable dischargeswitching condition is satisfied in the first stage for determination ofthe degree of ageing of the storage catalyst, the switching operatingpoint is determined as a function of the instantaneous operatingtemperature at the instant of switching, and wherein the respectiveswitching operating point in a second stage for determining the degreeof ageing of the storage catalyst is compared to the definable storagecatalyst capacity field which runs over a temperature window, which isoptimized especially with respect to fuel consumption, and which isformed by a plurality of individual operating points for a new and anaged storage catalyst such that a switching operating point which lieswithin the storage catalyst capacity field does not constitute a failureto reach the minimum nitrogen oxide storage capacity, but the changerelative to the previous operating point as a measure of the ageing ofthe storage catalyst, and wherein a switching operating point whichdeparts from the storage catalyst capacity field conversely constitutesa failure to reach the minimum nitrogen oxide storage capacity.
 6. Theprocess as claimed in claim 5, wherein to establish the switchinginstant from the storage phase to the discharge phase the relativenitrogen oxide slip as the difference between the nitrogen oxide massflow which has flowed into the nitrogen oxide storage catalyst and thenitrogen oxide mass flow which has flowed out of the nitrogen oxidestorage catalyst is determined relative to the storage time such thatthe quotient of the integral values of the nitrogen oxide mass flowupstream and downstream from the nitrogen oxide storage catalyst ismoreover brought into a relative relationship to the definable degree ofconversion of the nitrogen oxide which is derived from the exhaust gasboundary value so that when this defined switching condition is present,switching from the storage phase to the discharge phase is carried outat the switching instant which has been optimized with respect to fuelconsumption and storage potential.
 7. The process as claimed in claim 5,wherein the storage catalyst capacity field is limited relative to thetemperature window on the one hand by a boundary line for a new storagecatalyst and on the other hand by a boundary line for an aged storagecatalyst which represents the boundary ageing state, the temperaturewindow comprising preferably temperature values between approximately200° C. and approximately 450° C.
 8. The process as claimed in claim 1,wherein the vehicle is a motor vehicle.
 9. A method for operating aninternal combustion engine of a vehicle comprising: operating saidengine in a first operating range comprising: operating said engine witha lean mixture which has an air excess and an oxygen excess; storingnitrogen oxides produced by said engine in a nitrogen oxide storagecatalyst; switching operation of the engine to a rich operating range,wherein the rich operating range comprising utilizing a shortage of air;discharging the nitrogen oxides from the nitrogen oxide storagecatalyst; and operating said engine in a second operating rangecomprising: running said engine with an essentially stoichiometrichomogenous mixture (lambda=1), observing a definable switching conditiondepending on an operation-dictated load requirement and/or rpmrequirement; switching from the lean operating range to a rich operatingrange, upon reaching the definable switching condition; blockingswitching from the rich operating range to the homogeneous operatingrange if the additional amount of fuel consumption for discharges in acertain, definable evaluation interval which extends over several leanoperating phases is greater than or equal to the reduced amount of fuelconsumption by lean operation in this evaluation interval definableblocking criterion; switching between the lean operating range and thehomogeneous operating range, if the additional amount of fuelconsumption for discharges in the evaluation interval is smaller thanthe reduced amount of fuel consumption by lean operation in thisevaluation interval, determining the reduced amount of fuel consumptionas a function of the raw mass flow value of the nitrogen oxide averagedover the evaluation interval, as a function of the amount of fuel savedwhich has been averaged over the evaluation time interval in the leanoperating phases which occur in the evaluation interval compared to thehomogeneous operating range phases in this evaluation interval, and as afunction of the time between two torque requirements which exceed adefinable load boundary value and/or rpm boundary value and which causedeparture from the lean operating range, which time has been averagedover the evaluation interval, and determining the additional amount offuel consumption as a function of a storage catalyst charging stateaveraged over the evaluation interval.
 10. The method of claim 9,comprising using an engine control device to perform said switchingduring said second operating range.
 11. The method of claim 9,comprising using an engine control device to perform said switchingduring said first operating range.
 12. A method for operating aninternal combustion engine of a vehicle comprising: running said enginewith an essentially stoichiometric homogenous mixture (lambda=1),observing a definable switching condition depending on anoperation-dictated load requirement and/or rpm requirement; switchingfrom a lean operating range, having an air excess and an oxygen excess,to a rich operating range, having a shortage of air, upon reaching thedefinable switching condition, then to a homogenous operating range;blocking switching from the rich operating range to the homogeneousoperating range if the additional amount of fuel consumption fordischarges in a certain, definable evaluation interval which extendsover several lean operating phases is greater than or equal to thereduced amount of fuel consumption by lean operation in this evaluationinterval definable blocking criterion; switching between the leanoperating range and the homogeneous operating range, if the additionalamount of fuel consumption for discharges in the evaluation interval issmaller than the reduced amount of fuel consumption by lean operation inthis evaluation interval, determining the reduced amount of fuelconsumption as a function of the raw mass flow value of the nitrogenoxide averaged over the evaluation interval, as a function of the amountof fuel saved which has been averaged over the evaluation time intervalin the lean operating phases which occur in the evaluation intervalcompared to the homogeneous operating range phases in this evaluationinterval, and as a function of the time between two torque requirementswhich exceed a definable load boundary value and/or rpm boundary valueand which cause departure from the lean operating range, which time hasbeen averaged over the evaluation interval, and determining theadditional amount of fuel consumption as a function of a storagecatalyst charging state averaged over the evaluation interval.